1. Field of the Invention
The present invention relates to a valve that responds to the presence of a particular substance in fluid flowing through the valve, such as the presence of hydrocarbon in a flow of water.
2. Description of the Related Art
In the petroleum industry, it is often possible for hydrocarbon to leak into a flow of water that is discharged into the environment. For example, a hydrocarbon storage tank (such as is found in a "tank farm") often has an internal drain pipe to allow drainage of water from a fixed or floating roof of the tank. It is possible for an internal drain pipe to leak, causing stored hydrocarbon to drain from the tank. To prevent the contents of the tank from being drained to the atmosphere or ground, the discharge end of the drain pipe should be fitted with a hydrocarbon sensing automatic shutoff drain valve.
Hydrocarbon sensing automatic shutoff drain valves are described in Fraylick et al. U.S. Pat. No. 3,491,911 issued Jan. 27, 1970, and Thiltgen U.S. Pat. No. 5,036,875 issued Aug. 6, 1991, incorporated herein by reference. These automatic shutoff drain valves employ a hydrocarbon soluble triggering element disposed in the discharge flow, and a spring that closes the valve upon dissolution of the triggering element. The valves are intended to close in response to a relatively small trickle of water containing hydrocarbon. However, the flow through the open valve should not be unduly restricted or else clogging may occur from debris which are often present in drainage from tank roofs. The valves of Fraylick and Thiltgen have rather complex designs attempting address these problems.